1. Field of the Invention
The present invention relates to an electrophotographic image forming device such as copying machine, printer, or facsimile machine. More particularly, the invention relates to a charging device for uniformly charging a photoconductor using a corona discharge.
2. Description of the Related Art
Electrophotographic technology using dry toner is applied to various kinds of image forming devices such as copying machine, printer or facsimile machine due to inexpensive page cost.
FIG. 1 shows a general structure of an electrophotographic image forming device 100. The image forming device 100 includes a photoconductor 2 that is of a drum-shaped and is rotatably supported. A charging device 1, an exposing device 3, a developing device 4, a transfer device 5, and a cleaning device 6 are disposed along the periphery of the photoconductor 2. The charging device 1 uniformly charges the peripheral surface of the photoconductor 2 to negative polarity. The exposing device 3 irradiates light modulated in accordance with image data onto the peripheral surface of the photoconductor 2, thereby forming an electrostatic latent image. The developing device 4 supplies toner to the peripheral surface of the photoconductor 2 to develop the electrostatic latent image. A visible toner image is thus formed on the photoconductor 2, which is then transferred onto a recording medium 7, such as paper, by the transfer device 5. The cleaning device 6 cleans residual toner remaining on the photoconductor 2. The toner image transferred onto the recording medium 7 is conveyed to a fixing device 8 where the toner image is thermally fixed.
For the structural simplicity, a corotron or a scorotron has been used in the charging device 1. The corotron is configured from corona electrodes for generating corona discharges, and a U-shaped shield case for stabilizing the corona discharge. Application of a high voltage across the corona electrodes generates the corona discharge. The peripheral surface of the photoconductor 2 is electrostatically charged with ions generated when the corona discharge has occurred. The scorotron is similar in structure to the corotron but is additionally provided with a grid. The scorotron is disposed so that the grid is interposed between the photoconductor 2 and the corona electrodes. The grid serves to uniformly charge the peripheral surface of the photoconductor 2. In the absence of the grid, the peripheral surface of the photoconductor 2 tends to be non-uniformly charged because the corona discharge of negative polarity is not stable.
FIG. 2 shows a conventional charging device 1. The charging device 1 is disposed above the peripheral surface of the photoconductor 2 in confronting relation therewith. The charging device 1 includes a shield case 12, a partitioning plate 14, wire electrodes 15, and the grid 13.
The wire electrodes 15 are formed from a tungsten wire having a diameter in a range from 50 to 100 μm, and the surface of the tungsten wire is plated with gold to a thickness of several microns. Application of a high voltage, e.g., +/−5 kV, across the wire electrodes 15 results in generation of corona discharges. The photoconductor 2 is charged with ions generated resulting from the corona discharges.
However, corona discharges that generate negative polarity ions produce a great deal of ozone, which degrades electrical characteristics of the photoconductor 2 and also exerts bad influence upon living bodies. An amount of ozone produced by a tandem type color image forming device increases much more than that generated by a monochromatic image forming device, because the color image forming device includes a plurality of image forming unit for each color toner. Accordingly, an enhanced ozone discharging mechanism is required for the color image forming device. This, however, invites increase of cost and production of larger noises
In order to solve the above-described problems, Japanese Patent Application Publication No. 63-15272 proposes a charging device in which a saw-tooth shaped electrode is used in place of the wire electrode. The charging device with the saw-tooth shaped electrode decreases the amount of ozone to about one fourth (¼) as compared with the charging device using the wire electrode.
However, the corona discharges generated at the tip end portions of the saw-tooth electrodes attract foreign materials, such as dust, toner, or paper dust floating in the air. Once the foreign materials are adhered to the tip end portion of the saw-tooth electrode, the corona discharge tends to be unstable, causing the image to degrade. To stabilize the generation of corona discharges, the voltage applied to the saw-tooth shaped electrode needs to be increased. However, if the voltage applied to the saw-tooth shaped electrode becomes too high, the corona discharge proceeds to a spark discharge, and thus the photoconductor is damaged.
These problems can be solved if the saw-tooth electrode is perfectly cleaned. However, due to the sharpness of the tip end portion of the saw-tooth electrode, the cleaning member is easily damaged and so the cleaning effect is lowered.